Thursday, January 31, 2008

Test: The RJ45-DB9 Connector

Once you have successfully soldered your wires, and snapped the DB9 end onto the rest of the connector, test it. Incorrect wiring, or a short, can damage the hardware that you are using. You can test your connector with a multimeter. To make the testing easier, use an unconnected male DB9 end, that has metal tabs on the end for soldering. Plug the male end into the connector, leaving DB9 male soldering ends exposed. With the multimeter, set it to test for connectivity. If there is no connectivity setting, set the multimeter to test for resistance. Clearly, if you have a good connection, the multimeter will either beep (if you have a connectivity mode with beeper) or read zero (if you use the resistance mode).

Even with the solder male DB9 end, testing the device with the two probes can be challenge. You have to reference the chart on Ossmann's website, and make sure you stick the probes on the correct respective RJ45 and DB9 ends. That can be especially challenging, since DB9 end may not be numbered, and the RJ45 is most likely not numbered.

Start with the easy ends, like DB9 pin 1 to RJ45 pin 8. They are the first (and last) pins respectively. If you find the connection, you can then easily find the pin ordering, and test from there. The hardest part is keeping your hand still over the correct RJ45 pin, as test each connection one by one. Test both ends, by holding one probe over one pin, and making sure that only the correct pins on the other side connect.

Since the RJ45 side can be hard to test for rapidly, you might crimp another RJ45 plug, and leave the other end of wire uncrimped. Strip each skinny copper wire until there is enough exposed copper to allow simple contact with the probe.

Fortunately, this connector tested well, is ready to move onto a real hardware test.

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